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Frequently Asked Questions About Color

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Frequently Asked Questions About Color Frequently Asked Questions about Color Charles A. Poynton This FAQ is intended to clarify aspects of color that are important to color image coding, computer graphics, image processing, video, and the www.inforamp.net/~poynton transfer of digital images to print. [email protected] I assume that you are familiar with intensity, luminance (CIE Y), light- ness (CIE L*), and the nonlinear relationship between CRT voltage and intensity (gamma). To learn more about these topics, please read the companion Frequently Asked Questions about Gamma before starting this. This document is available on the Internet from Toronto at <ftp:// ftp.inforamp.net/pub/users/poynton/doc/colour/ColorFAQ.pdf> I retain copyright to this note. You have permission to use it, but you may not publish it. Table of Contents 1 What is color? 3 2 What is intensity? 3 3 What is luminance? 3 4 What is lightness? 4 5 What is hue? 4 6 What is saturation? 4 7 How is color specified? 4 8 Should I use a color specification system for image data? 5 9 What weighting of red, green and blue corresponds to brightness? 5 10 Can blue be assigned fewer bits than red or green? 6 11 What is “luma”? 6 12 What are CIE XYZ components? 7 13 Does my scanner use the CIE spectral curves? 7 14 What are CIE x and y chromaticity coordinates? 7 15 What is white? 8 16 What is color temperature? 8 17 How can I characterize red, green and blue? 9 1997-03-02 Charles A. Poynton. All rights reserved. 1 of 24 2 Frequently Asked Questions About Colour 18 How do I transform between CIE XYZ and a particular set of RGB primaries? 9 19 Is RGB always device-dependent? 10 20 How do I transform data from one set of RGB primaries to another? 10 21 Should I use RGB or XYZ for image synthesis? 11 22 What is subtractive color? 11 23 Why did my grade three teacher tell me that the primaries are red, yellow and blue? 11 24 Is CMY just one-minus-RGB? 12 25 Why does offset printing use black ink in addition to CMY? 12 26 What are color differences? 13 27 How do I obtain color difference components from tristimulus values? 14 28 How do I encode Y'PBPR components? 14 29 How do I encode Y'CBCR components from R'G'B' in [0, +1]? 15 30 How do I encode Y'CBCR components from computer R'G'B' ? 15 31 How do I encode Y'CBCR components from studio video? 16 32 How do I decode R'G'B' from PhotoYCC? 17 33 Will you tell me how to decode Y'UV and Y'IQ? 17 34 How should I test my encoders and decoders? 17 35 What is perceptual uniformity? 18 36 What are HSB and HLS? 19 37 What is true color? 19 38 What is indexed color? 20 39 I want to visualize a scalar function of two variables. Should I use RGB values corresponding to the colors of the rainbow? 21 40 What is dithering? 21 41 How does halftoning relate to color? 21 42 What’s a color management system? 22 43 How does a CMS know about particular devices? 22 44 Is a color management system useful for color specification? 22 45 I’m not a color expert. What parameters should I use to code my images? 23 46 References 23 47 Contributors 24 2 Frequently Asked Questions About Color 3 1 What is color? Color is the perceptual result of light in the visible region of the spectrum, having wavelengths in the region of 400 nm to 700 nm, incident upon the retina. Physical power (or radiance) is expressed in a spectral power distri- bution (SPD), often in 31 components each representing a 10 nm band. The human retina has three types of color photoreceptor cone cells, which respond to incident radiation with somewhat different spectral response curves. A fourth type of photoreceptor cell, the rod, is also present in the retina. Rods are effective only at extremely low light levels (colloquially, night vision), and although important for vision play no role in image reproduction. Because there are exactly three types of color photoreceptor, three numer- ical components are necessary and sufficient to describe a color, providing that appropriate spectral weighting functions are used. This is the concern of the science of colorimetry. In 1931, the Commission Interna- tionale de L’Éclairage (CIE) adopted standard curves for a hypothetical Standard Observer. These curves specify how an SPD can be transformed into a set of three numbers that specifies a color. The CIE system is immediately and almost universally applicable to self- luminous sources and displays. However the colors produced by reflec- tive systems such as photography, printing or paint are a function not only of the colorants but also of the SPD of the ambient illumination. If your application has a strong dependence upon the spectrum of the illu- minant, you may have to resort to spectral matching. Sir Isaac Newton said, “Indeed rays, properly expressed, are not colored.” SPDs exist in the physical world, but colour exists only in the eye and the brain. Berlin and Kay [1] state that although different languages encode in their vocabularies different numbers of basic color categories, a total universal inventory of exactly eleven basic color categories exists from which the eleven or fewer basic color terms of any given language are always drawn. The eleven basic color categories are WHITE, BLACK, RED, GREEN, YELLOW, BLUE, BROWN, PURPLE, PINK, ORANGE, and GRAY. 2 What is intensity? Intensity is a measure over some interval of the electromagnetic spectrum of the flow of power that is radiated from, or incident on, a surface. Inten- sity is what I call a linear-light measure, expressed in units such as watts per square meter. The voltages presented to a CRT monitor control the intensities of the color components, but in a nonlinear manner. CRT voltages are not proportional to intensity. 3 What is luminance? Brightness is defined by the CIE as the attribute of a visual sensation according to which an area appears to emit more or less light. Because bright- ness perception is very complex, the CIE defined a more tractable quan- tity luminance which is radiant power weighted by a spectral sensitivity function that is characteristic of vision. The luminous efficiency of the Stan- dard Observer is defined numerically, is everywhere positive, and peaks 4 Frequently Asked Questions About Color at about 555 nm. When an SPD is integrated using this curve as a weighting function, the result is CIE luminance, denoted Y. The magnitude of luminance is proportional to physical power. In that sense it is like intensity. But the spectral composition of luminance is related to the brightness sensitivity of human vision. Strictly speaking, luminance should be expressed in a unit such as candelas per meter squared, but in practice it is often normalized to 1 or 100 units with respect to the luminance of a specified or implied white reference. For example, a studio broadcast monitor has a white reference whose luminance is about 80 cd•m-2, and Y = 1 refers to this value. 4 What is lightness? Human vision has a nonlinear perceptual response to brightness: a source having a luminance only 18% of a reference luminance appears about half as bright. The perceptual response to luminance is called Lightness. It is denoted L* and is defined by the CIE as a modified cube root of lumi- nance: 1 Y 3 Y L*=116−< 16; 0. 008856 YnnY Yn is the luminance of the white reference. If you normalize luminance to reference white then you need not compute the fraction. The CIE defini- tion applies a linear segment with a slope of 903.3 near black, for ≤ (Y/Yn) 0.008856. The linear segment is unimportant for practical purposes but if you don’t use it, make sure that you limit L* at zero. L* has a range of 0 to 100, and a “delta L-star” of unity is taken to be roughly the threshold of visibility. Stated differently, lightness perception is roughly logarithmic. An observer can detect an intensity difference between two patches when their intensities differ by more than one about percent. Video systems approximate the lightness response of vision using R’G’B’ signals that are each subject to a 0.45 power function. This is comparable to the 1⁄ 3 power function defined by L*. 5 What is hue? According to the CIE [2], hue is the attribute of a visual sensation according to which an area appears to be similar to one of the perceived colours, red, yellow, green and bue, or a combination of two of them. Roughly speaking, if the dominant wavelength of an SPD shifts, the hue of the associated color will shift. 6 What is saturation? Again from the CIE, saturation is the colourfulness of an area judged in propor- tion to its brightness. Saturation runs from neutral gray through pastel to saturated colors. Roughly speaking, the more an SPD is concentrated at one wavelength, the more saturated will be the associated color. You can desaturate a color by adding light that contains power at all wavelengths. 7 How is color specified? The CIE system defines how to map an SPD to a triple of numerical components that are the mathematical coordinates of color space. Their function is analagous to coordinates on a map. Cartographers have different map projections for different functions: some map projections preserve areas, others show latitudes and longitudes as straight lines.
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